Typically, linear motion devices include an output element, such as a rod, which exerts a force in a linear direction to actuate a control or other device in an electro-mechanical system. In each particular application, it is important to design the linear motion device such that the output element has linear velocity and linear position operating characteristics suitable for the intended operating environment of the device. Several testing methods and apparatuses have been proposed heretofore to enable a designer to measure and evaluate certain operating characteristics of linear motion devices. These prior methods include the use of a calibrated spring, the application of weighted loads and a servo control to apply loads to the device to be tested.
In the calibrated spring method, a calibrated coil spring, for example, is utilized to exert a pushing load against the output rod of a linear device under test. Inasmuch as the forces generated by the spring are known, the position data for the device under test may be observed after application of the known spring load. Although the calibrated spring method is a simple and inexpensive method for evaluating linear motion devices, spring force is not constant over distance and the natural resonances of the spring detract from accurate operation. Moreover, the calibrated spring method does not provide velocity data and position data must be obtained by external observation and measurement. Indeed, a tester is only able to measure characteristics upon device failure and may not determine operating characteristics as a device approaches failure.
Pursuant to the weight approach, weight elements are used to apply a force to the output rod of a linear motion device under test and a position sensor is used to measure the weight element effects upon the position of the output rod. In order to increase the force applied to the device under test, additional weights must be added. The additional weights increase the mass applied as well as the force. Thus, the weight approach varies two variables simultaneously, i.e. mass and force and is not suitable for testing procedures wherein a specific fixed mass application is contemplated for the device under test.
In accordance with the servo control proposal, a servo control is used to apply a force to the output rod of the device under test. The use of the servo control eliminates the effects of mass and thus has the disadvantage of not permitting an evaluation of the device under test as it will operate in an actual system wherein mass has an effect upon operation. Moreover, the servo control adds considerable expense to this approach.